This invention relates to an accelerometer and particularly to an accelerometer at which is made at least partially from silicon.
The conventional quartz and silicon accelerometers as shown in FIG. 1 of the accompanying drawings have a bulk machined (wet etched) quartz or silicon structure which is thinned along a line to create a hinge 1, with a proof mass 2, on one side thereof in the form of a pendulum. The structure is normally implemented in the form of a sandwich with two fixed capacitor plates 3, located on either side of the moveable proof mass 2 which is hinged at 1 from a support 4 on which the capacitor plates 3 are mounted. With an acceleration in the direction A in FIG. 1 which is perpendicular to the plane of the silicon or quartz wafer and proof mass 2, the proof mass is deflected arcuately about the hinge 1 by an amount proportional to acceleration. This movement is usually sensed electrostatically and a restoring force is applied to return the proof mass 2 to a null position conveniently by use of electromagnetic feedback using a wire wound coil on the proof mass. Electrostatic force may also be used for the feedback. Feedback improves the scale factor linearity at high forces as the proof mass does not move. High accuracy accelerometers are usually closed loop.
Such a conventional accelerometer can provide high accuracy over a high gravity range but is generally expensive to manufacture and of relatively large size. Additionally as the structure is a pendulous structure the motion of the proof mass 2 is arcuate which gives rise to vibro pendulosity which introduces a cross axis sensitivity under a vibration. An additional problem which is common to this and other types of conventional accelerometers is vibration rectification. This means that in the presence of a vibration but no static gravity load such a conventional accelerometer can provide an erroneous output signal which is due to an imbalance between the two capacitor plates 3 which sense the deflection of the proof mass 2 and act to give an electrostatic restoring force.
The conventional accelerometer as shown in FIG. 1 requires the two capacitor plates 3 to be operated in anti-phase with the difference in capacitance value then being linearly proportional to the offset position and acceleration. An electrostatic force is supplied for feedback. This gives rise to three particular disadvantages. Firstly the electrostatic forces are quadratic in voltage so it is necessary to linearise the force which is proportional to acceleration. This can be difficult to do for precision accelerometers. Secondly, the conventional silicon accelerometer of FIG. 1 is pendulous which means that the proof mass 2 moves in a curved arc as a function of increasing acceleration. This arcuate motion means that when the proof mass 2 is away from the null position there is a sensitivity at right angles to the main sensing axis. This effect which is generally called vibro-pendulosity is an error which is particularly apparent for vibration when it is applied to excite both axis. At high frequency of movement the proof mass is not correctly restored to the null position. Thirdly, any offset between the values of the two capacitor plates 3 which are used differentially to detect movement of the proof mass 2 away from the null position can cause the vibration rectification effect as well as bias (zero offset). Hence it is necessary accurately to match the values of the two capacitor plates 3 which is difficult to do with the sandwich structure of FIG. 1. Accordingly electronic offsets are typically used to null out any imbalance which is not desirable as any drift in this nulling signal will cause a drift in the accelerometer bias, which is a key parameter to keep stable.
A second type of conventional accelerometer is shown in FIG. 2 of the accompanying drawings which uses vibrating beams 5, the frequency of vibration of which varies with strain and acceleration. The vibrating beams 5 are attached to a proof mass 6 so that the acceleratative force on the proof mass 6 changes the strain on the vibrating beams 5, either in compression or tension, to give a frequency output which varies with the gravitational force. The vibrating beams 5 generally operate differentially so that one side is in compression and the other is in tension with a frequency increase and decrease respectively of the beams. The difference frequency is then a good measure of acceleration. Movement of the proof mass is in a sensing direction B as shown in FIG. 2, with the vibrating beams 5 in effect suspending the proof mass 6 between two mounting supports 7.
The conventional accelerometer of the type shown in FIG. 2 can use quartz for the vibrating beams 5 and proof mass 6. Such accelerometers can be made smaller and slightly cheaper than the pendulum type accelerometer of FIG. 1 but they are still considerably expensive to manufacture, and are open loop accelerometers which do not usually have force feedback and which may be subject to linearity errors at high input accelerations.
There is thus a need for an improved accelerometer which utilises silicon and which at least minimises the foregoing difficulties inherent in the conventional accelerometers illustrated in FIGS. 1 and 2.
According to one aspect of the present invention there is provided an accelerometer having a substantially planar plate-like proof mass, four or more flexible mounting legs each co-planar with the proof mass, a substantially planar, ring-like support, in which the proof mass is movably mounted, which support is fixedly mounted relative to the proof mass and co-planar therewith, with each mounting leg being connected at one end to the proof mass and connected at another end to the support so that the proof mass is mounted for linear movement in a sensing direction in the plane containing the proof mass, mounting legs and support, in response to acceleration change applied to the accelerometer, and with the mounting legs extending substantially perpendicularly to the sensing direction, at least two spaced apart substantially planar capacitor plates, mounted in the ring-like support substantially transverse to the sensing direction with the proof mass located between the capacitor plates and with each capacitor plate being coplanar with the proof mass, mounting legs and support, for sensing linear movement of the proof mass in the sensing direction, a plurality of interdigitated fingers in air, comprising first arrays of laterally spaced fingers extending substantially perpendicularly to the sensing direction from the support towards the proof mass and second arrays of laterally spaced fingers extending substantially perpendicularly to the sensing direction from the proof mass towards the support, with the first arrays of fingers being interdigitated with the adjacent second arrays of fingers to provide air squeeze damping for movement of the proof mass in the sensing direction relative to the support, with the proof mass, mounting legs, support capacitor plates and interdigitated fingers are formed from a single plate of silicon, and restoring means for returning the proof mass in the sensing direction towards a null position.
Preferably the proof mass, mounting legs, support capacitor plates and interdigitated fingers are formed by dry etching from a plate of silicon which is orientated in a [111] or [100] crystal plane.
Conveniently the support has a substantially rectangular ring-like shape surrounding an inner open area in which is located the proof mass which has a substantially rectangular shape, and wherein the mounting legs extend substantially perpendicularly to the sensing direction in spaced array, with at least two between a first inner wall of the support defining the inner open area and a facing first outer wall of the proof mass and with at least two between the opposing second inner wall of the support defining the inner open area and the facing second outer wall of the proof mass.
Advantageously the mounting legs have high compliance in the sensing direction and low compliance in other directions.
Preferably the accelerometer includes a support sheet of non-conductive material on which is fixedly mounted the support and capacitor plates, with the mounting legs, proof mass and interdigitated fingers being spaced from the support sheet.
Conveniently the support sheet is made of glass to which the support and capacitor plates are fixedly mounted by anodic bonding.
Advantageously the restoring means is electromagnetic.
Preferably the proof mass carries a thin film electrically conductive coil structure, with each mounting leg carrying at least part of a turn of the coil structure.
Conveniently the capacitor plates are located in the inner open area of the support.
Advantageously the accelerometer includes an apertured glass frame located on the side of the support sheet remote from the support, and attached to the support sheet, and the restoring means includes a magnet and two bar pole pieces located in the glass frame aperture with the pole pieces being spaced apart at opposite ends of the magnet in register with the coil structure turns carried on the proof mass.
Preferably the adjacent surface of the support sheet is recessed to receive part of the bar pole pieces and a link pole piece is located on the support side of the support sheet to extend between and in registry with the two bar pole pieces.
Conveniently the accelerometer includes a sheet-like glass base attached to the glass frame on the side thereof remote from the support sheet to retain the magnet and bar pole pieces within the glass frame aperture, and a glass housing for the link pole piece.
Advantageously the magnet is poled in the sensing direction.
Preferably the first arrays of laterally spaced fingers extend substantially perpendicularly to the sensing direction from said first and second inner walls of the support towards the adjacent first and second outer walls of the proof mass and the second arrays of laterally spaced fingers extend substantially perpendicularly to the sensing direction from said first and second outer walls of the proof mass towards the adjacent first and second inner walls of the support.
Preferably the accelerometer includes at least two earth screens located within the inner open area, each between the adjacent capacitor plate and the adjacent third or fourth inner wall of the support defining the inner open area, and operable to shield the capacitor plates from the support, with the capacitor plates being electrically insulated from the earth screens and with the earth screens being electrically insulated from the support on which said earth screens are mounted.
Conveniently the drive means includes means for supplying a square wave drive voltage to the two capacitor plates in anti-phase.
Advantageously the accelerometer includes means for supplying a control current to the proof mass coil structure, which control current supply means includes a pre-amplifier for receiving from the proof mass coil structure a difference signal between the two capacitor plates at Alternating Current (AC) modulation frequency resulting from imbalance of the capacitor plates under acceleration, an AC demodulator for synchronously demodulating the output signal from the pre-amplifier, an integrator for integrating the output signal from the demodulator, a loop filter for ensuring stability of the output signal received from the integrator and a current driver for receiving the output signal from the loop filter and for feeding a control current to the proof mass coil structure.
Preferably the drive means includes means for supplying a single modulation Alternating Current (AC) to the proof mass, which gives rise to an output signal from each of the capacitor plates.
Alternatively the means for supplying a control current to the proof mass coil structure includes two pre-amplifiers, one for receiving the output signal from one of the capacitor plates and the other for receiving the output signal from the other of the capacitor plates, a differential amplifier for receiving the output signals from the two pre-amplifiers and for differencing the pre-amplifier output signals to give an output signal corresponding to the net displacement of the proof mass, and an AC demodulator for receiving the output signal from the differential amplifier and for synchronously demodulating the output signal from the differential amplifier to produce a Direct Current (DC) output signal for passage as a control current to the proof mass coil structure to return the proof mass in the sensing direction towards a null position.